Google plans to offer an additional 5.3 billion shares through a flotation in April that will raise an estimated $2.1bn. 

"The principal purpose of this offering is to obtain additional capital," Google said in a filing with the Securities and Exchange Commission. 

The company explained that the funds might be used for future acquisitions, but that there are no current agreements or commitments for such activity. The transaction is expected to be completed next month.

Google's move was in part sparked by its forthcoming inclusion in the Standard & Poor's 500 Index at the end of March.

The search giant anticipates that this will cause many index funds to purchase Google stock, and the flotation is designed to meet the additional demand.

Google has raised $5.7bn from investors so far. The company went public in August 2004 and sold an additional $4bn in stock last August.

Google revealed plans for the stock sale after the market had closed on Wednesday. The company's share price fell several percentage points in after-hours trading.